1. Field of the Invention
The present invention relates to a dielectric ceramic and a laminated ceramic capacitor, and particularly, relates to a dielectric ceramic which is suitable for reduction in size and increase in capacitance of laminated ceramic capacitors, and a laminated ceramic capacitor configured with the use of the dielectric ceramic.
2. Description of the Related Art
One of effective means for satisfying the needs of reduction in size and increase in capacitance for laminated ceramic capacitors is to reduce the thickness of dielectric ceramic layers provided in the laminated ceramic capacitors. However, the electric field strength applied to the dielectric ceramic layers is increased as the reduction in thickness of dielectric ceramic layers is promoted. Therefore, there is a possibility of a decrease in withstand voltage or deceased reliability against high-temperature and high-field load tests in the laminated ceramic capacitors.
In order to solve the problem described above, it is effective to reduce the size of the ceramic grains constituting the dielectric ceramic layers. However, the reduction in size of the ceramic grains causes another problem as it leads to a decrease in the dielectric constant of the dielectric ceramic.
Under these circumstances, the International publication WO2006/018928 (Patent Document 1) discloses a dielectric ceramic which is capable of obtaining a dielectric constant of 3000 or more while ensuring reliability in the withstand voltage and load test, even when dielectric ceramic layers are reduced in thickness.
Specifically, Patent Document 1 discloses a dielectric ceramic represented by the general formula: {100(Ba1-x-ySrxCay)m(Ti1-zZrz)O3+aBaO+bR2O3+cMgO+dMnO+eCuO+fV2O5+gXuOv} (provided that R represents at least one selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y, and XuOv represents a group of oxides containing at least Si),
where 0≦x≦0.05; 0≦y≦0.08; 0≦z≦0.05;
0.990≦m; 100.2≦(100m+a)≦102;
0.05≦b≦0.5; 0.05≦c≦2; 0.05≦d≦1.3;
0.1≦e≦1.0; 0.02≦f≦0.15; and 0.2≦g≦2.
One of the features of the dielectric ceramic which has the composition described above is the relatively low R content (b) of 0.05 to 0.5 in terms of R2O3 with respect to 100(Ba,Sr,Ca)m(Ti,Zr)O3. In addition, an example in Patent Document 1 mentions that it has been confirmed that almost no grain growth caused by firing is observed in the sintered body obtained from the dielectric ceramic which has the composition described above and the ceramic sintered body has almost the same average grain size as the average grain size of the main constituent of the ceramic raw material.
Laminated ceramic capacitors configured with the use of the dielectric ceramic described in Patent Document 1 have low reliability against changes in temperature and mechanical shocks, and thus have a problem that breakages or cracks are likely to occur, thereby leading to a defect in withstand voltage and a decrease in insulation resistance. Therefore, defects may decrease the yield in the case of mounting the laminated ceramic capacitors onto desired substrates. In addition, when the laminated ceramic capacitors are placed in particularly severe environments, for example, as in the case of in-car applications, the laminated ceramic capacitors may fail to adequately cope with such environments.
[Patent Document 1] International publication WO2006/018928